CENP-A/CENP-B uncoupling in the evolutionary reshuffling of centromeres

Background While CENP-A is the epigenetic determinant of the centromeric function, the role of CENP-B, the sole centromeric protein binding a specific DNA sequence (CENP-B-box), remains elusive. In the few mammalian species analyzed so far, the CENP-B box is contained in the major satellite repeat that is present at all centromeres. We previously demonstrated that, in the genus Equus , some centromeres lack any satellite repeat. Results Here, we show that, in four Equus species, CENP-B is expressed but does not bind the numerous satellite-free and the majority of satellite-based centromeres while it is localized at several ancestral now inactive centromeres. The absence of CENP-B is related to the lack of CENP-B boxes rather than to peculiar features of the protein itself. CENP-B boxes are comprised in a previously undescribed repeat which is not the major satellite bound by CENP-A. Comparative sequence analysis suggests that this satellite was centromeric in the equid ancestor, lost centromeric function during evolution and gave rise to a short CENP-A bound repeat not containing the CENP-B box but being enriched in dyad symmetries. Centromeres lacking CENP-B are functional and recruit normal amounts of the centromeric proteins CENP-A and CENP-C. Conclusions We propose that the uncoupling between CENP-B and CENP-A may have played a role in the evolutionary reshuffling of equid centromeres. This study provides new insights into the complexity of centromere organization in a largely biodiverse world where the majority of mammalian species still have to be studied. ### Competing Interest Statement The authors have declared no competing interest.